Circuits using a PWM method are widely used as circuits for driving regulated current or voltage in output stages. A PWM method is widely used in DC-DC converters. A PWM method is also used for various purposes, such as those of a buck/booster converter and a current regulator circuit, based on circuit topology.
In a PWM circuit, the ratio of the amount of charges transferred to an output stage and the amount of charges discharged from the output stage is controlled in response to the duty cycle of a control signal pulse. The voltage of the output stage reaches a regulated DC value depending on the balance between the amount of charges transferred to the capacitance of the output stage and accumulated thereon and the amount of charges discharged from the capacitance of the output stage. To control a duty cycle of PWM circuit, a closed looped control scheme based on feedback of the state value of an output stage is widely used. An open looped control scheme using an external signal, not feedback, may be used in special applications.
In general, in a regulated current driving circuit, a load, a driving transistor, and a resistor are connected in series, and undergo the process of converting DC voltage into DC current.
A multi-channel regulated current driving apparatus is widely used when the currents of light-emitting diodes (LEDs) are driven. LEDs need to be driven at constant current in order to keep the brightness thereof constant because the amount of light emitted by an LED is determined by a current value. To use LEDs as light sources, a plurality of LEDs is connected via multiple channels. Each of the LED channels includes a plurality of LEDs connected in series, and the plurality of LED channels is connected in parallel, thereby forming a multi-channel LED array.
In this case, the multi-channel regulated current driving apparatus controls the LED channels to flow the same current via the LED channels regardless of their loads. The general technical object of multi-channel regulated current driving apparatuses is to provide a control circuit having a small area and also effectively reduce the differences in regulated current levels between multi-channel loads. The differences in regulated current levels results from mainly due to the differences between the characteristics of loads, such as LEDs.
High currents flow into LEDs with forward bias voltage Vf for emitting light. Different current levels may flow into the LEDs due to the differences in characteristic between the LEDs even with respect to the same bias voltage. In a multi-channel regulated current driving apparatus, a plurality of LED channels can be driven in the state in which a single DC-DC converter circuit is connected to the plurality of LED channels in common in order to reduce the area of a control circuit. In this case, the output of the DC-DC converter is determined based on the LED array of an LED channel having a high forward bias voltage.
In a conventional multi-channel LED constant current control apparatus, excessive voltage may be applied to both ends of each transistor for regulated current driving due to the difference in forward bias voltage between LED channels. In this case, the transistor through which high current flows has high power consumption, and generates a large amount of heat. In order for the transistor to normally operate regardless of high power consumption and a large amount of heat, the size of the transistor should increase because the transistor requires a high-capacity channel (or a high channel ratio) in order to withstand the high power consumption. As a result, the transistors of the conventional multi-channel LED constant current control apparatus are not integrated into an integrated circuit (IC) due to their increased sizes, but are disposed outside an IC.
In order to mitigate the above problem, Korean Patent Application Publication No. 10-2013-0050509 entitled “Apparatus for Current Regulation of Multi-Channel LEDs and Liquid Crystal Display Using Same” discloses a technology for effectively controlling electric current.
The prior art of current regulator for multi-channel LEDs includes: a plurality of LED channels each including an LED array, a transistor and a variable emitter resistor; a feedback sensing circuit configured to sense the collector voltage of the transistor in each of the LED channels; and a control circuit configured to increase the variable emitter resistance of an LED channel, in which the collector voltage of the transistor is higher than those of other LED channels, in response to a feedback sensing result received from the feedback sensing circuit.
In the prior art, there has been introduced a means for monitoring electric current flowing into a driving transistor for each channel and then reducing the difference in regulated current levels between channels by increasing voltage applied to a variable resistor when excessive current flows into the channel. In particular, each channel can be limited such that excessive current does not flow into the driving transistor of the channel.
In the prior art, the rated currents of transistors can be lowered, the sizes of the transistors can be reduced, and the power of the transistors unnecessarily consumed in LED channels having low forward bias voltage can be reduced. Accordingly, the transistors can be integrated into an IC. Moreover, the prior art can reduce total system and circuit costs because the area of the printed circuit board (PCB) of a multi-channel LED device is reduced by integrating driving transistors into an IC.
However, the prior art has limited applications because it can be applied to only the cases where the driving current values of driving transistors can be predicted. Furthermore, in the prior art, the difference in current levels between driving transistors is removed by controlling the values of the variable resistor. If the difference in current levels between the driving transistors is very large, the variable resistor should have a very wide control range in order to deal with the large difference. Accordingly, a problem arises in that a circuit area required to implement such a variable resistor increases. Furthermore, another problem arises in that it is difficult to precisely control the currents of the driving transistors because the resolution of the current levels of the driving transistors can be determined by resolution of the variable resistor.
Therefore, there is a need for the development of a multi-channel current driving scheme having a smaller circuit area and various application fields and for the development of a multi-channel current driving apparatus capable of high-resolution control even in a small circuit area.